The second Internet-connection component that can slow down your Internet connection is the gateway. Typically, that’s a router, such as an AirPort base station, connected to your broadband modem.
Double-Check Your NAT: If your service is 30 Mbps or faster, and your gateway is configured to hand out private network addresses to your local network, that gateway might become a bottleneck for data.
The problem is that network address translation (NAT), which lets multiple devices on a local network share a single gateway IP address, requires constant computation. Every hunk of data passing in or out needs to have the addresses rewritten. If that data is coming in at high speeds, your gateway’s NAT may not be able to keep up.
If you have a superfast Internet account, the workaround is to pay for static IP addresses from your ISP and manually assign one to each of your Macs. You can then turn off NAT in your gateway.
Disable WDS: If you connect multiple base stations via Wi-Fi using the Wireless Distribution System (WDS)—in Apple base stations released starting in 2007, this functionality is called Extend A Wireless Network—you could be shortchanging your bandwidth. When WDS is turned on, each byte of data sent has to be retransmitted for each base station. So if you have two base stations, your network’s bandwidth is cut in half; if you have three, it’s cut in thirds; and so on. Disabling WDS and instead using Ethernet or powerline networking to connect your base stations should restore your full speed.
The biggest problem with modems is that they get old.
Make Sure Your Modem Is Modern: If you haven’t bought a broadband modem or received a new one from your ISP within the last three years, call your ISP to make sure the one you have is up-to-date enough to handle your connection.
Some early DSL modems still in use don’t support the faster speeds of newer DSL lines—their slower CPUs can throttle downloads. Newer hardware can work faster.
Similarly, older cable modems that support the DOCSIS 1.0 standard won’t be able to keep up with networks that encode data with DOCSIS 2.0 (or, as is increasingly the case, DOCSIS 3.0). Check your cable modem’s manual to see which standard is in use.
Your ISP might swap your old modem for a new one at no cost. If you’re renting the modem, you should (politely) demand that upgrade. If your initial contract term is up, you can often renew and get a new modem at no cost. If you’re within a contract term, suggest that you’re thinking of switching providers; you could very well get a new modem in response.
Check Your Connections: Because a broadband modem has one input for a phone line or a coaxial cable, a bad hunk of cable or a wobbly plug can mean inconsistent service, with frequent drops or slowdowns.
Watch the Lights: As with your gateway, keep an eye on your modem’s status lights if the network slows down. One DSL modem I had crashed every time a Mac connected to its Web interface, but I’d never have known that if I hadn’t been watching its status lights. If something like that happens, call your ISP to start troubleshooting. (Mine was unable to solve the problem; I ended up switching ISPs.)
The Wide Area Network
If you’ve checked your local network, gateway, and modem, and they all seem to be up to snuff, you should check one more potential source of Internet slowdowns: the wires between your modem and the Internet.
Check Your Cable Again: The cable from the back of your broadband modem to the nearest phone or cable wall jack can suffer from the same problems as any Ethernet cabling on your local network. Check it for bends and kinks, and make sure the insulation shows no signs of wear. If it does, replace it. With coaxial cable, make sure the hard-to-turn hexagonal nut around the connector is as tight as it can be. To be sure, lift the cable to release tension while turning the nut.
Call Your ISP: If you suspect that the line outside your house is causing connection problems, call your ISP and ask for a line test. Such tests aren’t as useful as they might be: they can detect only whether your modem is responding. (If you have DSL, your provider may also be able to check the impedance on the line and use that to figure out if it is achieving the proper signal-to-noise ratio for your line speed.)
Watch the Weather: At some point, a wire leaves your house and connects to a demarcation point at a utility pole or a buried line. That connection is typically exposed to the elements. If your bandwidth sags when the wind blows, call your ISP for a service call to check the wire. When service is erratic, calling your ISP is especially useful; a technician may be able to test the line and witness a problem. (Keep in mind that if the ISP doesn’t find a problem, you may have to pay for a service call.)
Call an Electrician: If you’re certain that there’s a problem between your modem and the Internet, but you’re also certain that the problem doesn’t lie between your modem and the wall jack or between your house and the ISP, you may have a problem inside your walls. (For example, I once had a wasps’ nest in an exterior wall, and the insects ate the insulation on the cable coming into the house.) Indoor wiring is your responsibility; if you’re sure that’s where the problem lies, you’ll have to either hire a professional (in some states, that means an electrician) or pull some new wire yourself.